This invention relates to a method and apparatus for geothermal energy production from hot dry rock reservoirs using supercritical fluids.
There have been developed various methods of extracting heat from dry geothermal (hot dry rock) reservoirs, such as that described in U.S. Pat. No. 3,786,858 (Potter, et al, Jan. 22, 1974). These methods rely upon water to hydraulically fracture formations to form the reservoirs.
Once a fractured reservoir has been formed, production wells are drilled to intersect the hot dry rock reservoir. Then water, used as the geofluid, is pumped into the reservoir through the injection well which was previously used for hydraulic fracturing during reservoir formation. The water flows across the fractured surfaces of the hot dry rock, is heated by contact with the hot rock, and then is used to transfer the geothermal heat to the surface by flowing upward through one or more production wells in a pressurized, closed-loop circulating operation referred to as heat mining.
At the surface, the heat contained in the circulating geofluid is transferred to a second fluid, referred to as a binary working fluid, in a high-pressure heat exchanger of conventional design, and then the cooled geofluid reinjected into the hot dry rock reservoir. This second fluid, which can also be water, is more commonly ammonia, or one of a class of halogenated hydrocarbon refrigerants such as the Freons(trademark), or one or a mixture of low molecular weight hydrocarbons such as isobutane or isopentane.
In common practice, even though the hot pressurized geofluid is relatively benign chemically, it is not flashed to steam at the surface because of the release to the environment of small amounts of environmentally undesirable dissolved materials such as hydrogen sulfide, boron, arsenic, fluorides and other trace minerals in the aqueous geofluid. More significant quantities of silica, chlorides, and carbonates are also typically dissolved in the aqueous geofluid, potentially causing corrosion and undesirable deposits on turbine blades and other metallic surfaces in power plants and in heat exchangers.
Water-based geothermal systems generally have a geochemically determined temperature limit controlled by the critical point of water (384xc2x0 C. and 22 MPa). As the critical point for water is reached and then surpassed, the enhanced dissolution of silica followed by retrograde precipitation above 384xc2x0 C. presents a substantial obstacle to operating a hot dry rock geothermal reservoir at higher than the critical temperature for water. For hot dry rock reservoirs created in the most common igneous and metamorphic rocks and mixtures of the most common igneous and metamorphic rocks, where silica is present as either a primary or secondary (i.e., fracture-filling) mineral, the silica dissolution and reprecipitation problem occurs as the critical temperature for water is exceeded. Although drilling systems are capable of reaching rock temperatures in excess of 400xc2x0 C., concerns about enhanced geochemical interactions arise in water-based hot dry rock geothermal energy systems at these temperatures.
Because of the excellent inorganic solvent properties of water and because of the slow diffusional flow of water through the microcrack porosity in underground reservoirs, it is often difficult to control the chemistry of the water used as the production geofluid.
Thus, there is still a need for improved methods of producing hot dry rock geothermal energy.
Therefore, it is an object of this invention to provide another method for production of geothermal energy.
It is another object of this invention to provide a method of producing geothermal energy with supercritical fluids.
It is a further object of this invention to provide a method of producing geothermal energy with supercritical carbon dioxide.
It is still another object of this invention to provide a method of producing geothermal energy with improved control of the geofluid chemistry.
It is yet another object of this invention to provide a method of sequestering carbon dioxide in deep rock formations.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims appended hereto are intended to cover all changes and modifications within the spirit and scope thereof.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there has been invented a method for producing geothermal energy from deep regions of hot, essentially dry rock using fluids other than water for creation of underground reservoirs, production of geothermal energy, and, optionally, as working fluids in power plants. Underground reservoirs are created by pumping a supercritical fluid such as carbon dioxide into a rock mass to fracture the rock. Once the confined geothermal reservoirs are formed, the same supercritical fluids are circulated into the geothermal reservoirs, are allowed to heat up and expand, and then are pumped out of the reservoir to transport the heat to surface power generating plants or other direct heating applications.